In a small saucepan, mix the cocoa powder, sugar, and flour or cornstarch together.

Gradually add the milk, stirring constantly to prevent lumps.

Place the saucepan over medium heat and stir continuously.

As soon as it starts to thicken, reduce the heat to low and cook for another 2 minutes, stirring constantly.

Serve the hot chocolate in small bowls and add a little decoration on top if you like.